1. Field of the Invention
The present invention relates to concentrated glycol suspensions of water-soluble agricultural materials having superior stability and pour properties, and that upon dilution in a suitable volume of water, are suitable for application to plants as herbicidal compositions, adjuvants in herbicidal compositions, or fertilizers, or that may be used for other purposes.
2. Description of the Related Art
When growing crops in a field, it is important to kill or control the growth of undesirable plants (weeds) in the field. If not controlled, the weeds compete with crop plants for essential resources such as soil nutrients, water and sunlight. By removing a fraction of the resources or otherwise reducing the availability of these resources to crop plants, the weeds restrict crop growth, resulting in loss of crop yield.
Timely and judicious use of herbicides can provide weed control to minimize crop losses and production costs. Herbicides such as glyphosate (N-phosphonomethyl glycine) and many others are useful for control of a large variety of weeds. When used in an herbicidal composition, glyphosate is generally in the form of one of its various salts in solution, preferably an aqueous solution.
Adjuvants are materials that enhance the action of herbicides by promoting adsorption and translocation and by complexing antagonistic metal ions in the water used to make the herbicide solution. Ammonium sulfate has been known as an adjuvant for several decades. It is perhaps the most important commercial adjuvant and is also widely used as a fertilizer.
Herbicides are typically applied to field crops by spraying an aqueous mixture of several components. Polymers that inhibit spray drift, defoamers, and other chemicals that enhance the performance of an herbicide are sometimes mixed with the ammonium sulfate. All components of a mixture are typically ground to a small particle size in order to reduce the time required to dissolve. Growers and contract applicators use a “mix” tank to prepare the herbicide mixture. A typical mix consists of about 800 pounds of water, 8 to 17 pounds of herbicide, and 8 to 17 pounds of dry adjuvant. The result is a 1% or 2% solution of herbicide and 1% or 2% of adjuvant. The typical tank has relatively poor mixing and the applicator, whether using a truck or airplane, usually has little time to wait for dissolution. Even small particles tend to fall to the bottom of the mix tank in clumps and are sometimes slow to dissolve. In addition, dry materials may generate undesirable dusty conditions, absorb moisture, tend to cake, and are difficult to meter.
Liquid preparations facilitate use. However, liquids may suffer disadvantages because of low product concentration. Agricultural materials of interest have limited water solubility. Freight, handling, and packaging costs can become a substantial part of the total product cost. As an example, the highest practical concentration of ammonium sulfate in aqueous solution is about 38%. This concentration drops to about 34% if drift retardant polymer is included. Water is the only practical solvent because ammonium sulfate is practically insoluble in all other common solvents. Thus, while aqueous solutions are not satisfactory, there remains a need for liquid agricultural products.
An alternative approach to a liquid product is to create a suspension. Many examples of aqueous suspensions of agricultural materials are known in the art. Japanese patents JP 05105606, JP 02104502, EP 253682, and U.S. Pat. No. 4,526,606 are exemplars. However, in aqueous suspensions of water-soluble materials, crystal dissolution and recrystallization occurs continually. This results in a progressive increase of the size of the particles and eventual setting. Therefore, it is desirable and convenient to be able to prepare high concentration suspensions of agricultural solids in liquids in which they are not appreciably soluble.
In order for solid materials, which are generally denser than liquids to remain suspended, they must be broken up into small particles. Generally, the smaller the particle the more stable the suspension. Colloidal particles of 0.001 mm (1 micron) or less in diameter form stable suspensions because of Brownian motion. However, the energy required to break down ammonium sulfate and most other materials to colloidal dimensions is extremely high and impractically costly.
The liquid in a suspension intended for agricultural purposes must comply with government regulations, and should be qualified for use with pesticides under the Code of Federal Regulations (C.F.R.). The liquid may be soluble or insoluble in water. Mineral oils and a few other oils such as soybean oil are approved for agricultural use under 40 C.F.R. 180.1001, paragraph c and have been used as adjuvants themselves. Petroleum distillates are also approved under this section of the C.F.R. provided they conform to the conditions of 21 C.F.R. 172.882 or 21 C.F.R. 172.884. Co-pending application Ser. No. 09/855.481 describes useful suspensions of agricultural materials in water-insoluble oils.
However, suspending liquids that are water-soluble may be advantageous in that they have the potential to facilitate dispersion and dissolution of suspended solids in the applicator's “mix” tank. Of importance to this invention, propylene glycol and dipropylene glycol are approved for agricultural use under paragraph c of 40 C.F.R. 180.1001, and ethylene glycol and diethylene glycol are approved under paragraph d of the same part.
It is not yet possible to predict what combination of liquid and solid will yield a stable suspension. There is little understanding of the fundamentals of suspensions. For instance, Yan et. al. state that. “the effect of particle size on the rheology of suspensions is a controversial subject” (Y. Yan, et. al., Chem. Eng. Sci., 46(4), 985-994, (1991)). Other authors have claimed that the shape and ionic character of the particles can be important factors. For instance, R. M. Turian, et. al. in a paper published in Powder Technology, 93, 219-223 (1997) suggests that the “interparticle interaction effects were quite strong” for the suspensions he investigated. It is to be expected that different particles may present uniquely different properties.
Problems with use of agricultural suspensions may include syneresis, inability to pour, and difficulty dissolving. To begin with, suspensions must be stable. Syneresis is a phenomenon of phase separation marked by free clear liquid separating from a suspension. It is undesirable because it results in an inhomogeneous mixture. The most convenient method of measuring syneresis is to measure the depth of the free liquid in a container after sitting for some period of time compared to the total depth of the suspension and expressing the result in terms of percentage or fraction.
The first step in using an agricultural suspension is pouring it from its shipping container into a mix tank to be diluted for application. It is necessary that the suspension have the ability to be conveniently and quantitatively transferred from one container to another under the influence of gravity, i.e. to have good pourability. Pourability and suspension stability may be competitive properties. As previously noted, suspension stability generally increases as particle size decreases. However, suspension viscosity tends to increase as particle size becomes smaller. Suspension stability also generally increases with a higher viscosity suspending liquid. With some choices of particle size and suspending liquid the viscosity can become so high that the suspension will no longer pour. A balance between stability and pourability must be achieved.
The solids in a suspension must be readily dispersed and/or dissolved in a mix tank. If a suspension is too viscous or otherwise antagonistic toward water, the suspension will remain as large globules in the water and the suspended solids will be slow to dissolve. Dispersion means that the globules of the suspension break up into small liquid droplets to form a milky volume in the water. This is sometimes referred to as “bloom”. The resulting high surface area allows the solid particles rapid access to water where they can dissolve quickly. Vigorous agitation can also result in good dispersion, but in mix tanks where agitation-is frequently marginal, the ability of a suspension to quickly and spontaneously “bloom” can significantly reduce dissolving times.
Suspensions are well known in the art although few have been reported that employ non-aqueous liquids. For instance, U.S. Pat. No. 3,793,015 and British Patent GB 1,151,141 reported stable suspensions of particles such as carbon black in aliphatic hydrocarbons using metal salts of pyrophosphates and similar agents as dispersing agents. The suspensions were described as useful in imaging such as xerography. U.S. Pat. No. 3,317,635 reported dispersions of polymer particles in organic liquids. These are different than conventional suspensions of solid materials since they are prepared by in situ polymerization of aqueous liquid droplets in a hydrocarbon fluid. U.S. Pat. No. 4,393,151 described suspensions of water-soluble polymers in a liquid hydrocarbon medium including a thickening agent. German Patent 2312372 described suspensions of polishing agents such as alumina, zirconia and silicon carbide in ethylene glycol thickened with a neutralized carboxymethylene resin. These suspension were not designed to be dispersible in water.
Surfactants and soluble polymers are often used to stabilize suspensions of particles. These may function by adsorption on the particle surface and may provide steric interference that inhibits settling. However, the interaction between a particular liquid, particle, and surfactant is impossible to predict.
Suspensions of water-soluble solids in non-aqueous water miscible fluids such as glycols, ketones, and alcohols are described in U.S. Pat. Nos. 5,707,551 and 5,906,962. Specific examples are suspensions of potassium nitrate and of ammonium dihydrogen phosphate in propylene glycol and diethylene glycol. Stability of the suspensions is obtained through use of a surfactant system having three necessary components and the possible inclusion of water. U.S. Pat. No. 5,906,962 describes a similar surfactant system additionally containing a water hydratable polysaccharide.
The first necessary component of U.S. Pat. No. 5,707,551 is a nonionic polymeric “viscosity improver”, preferably a polymeric material with a volume mass of less than 15,000. The patent states that the “viscosity improver” increases viscosity. Most preferably, the “viscosity improvers” are ethylene-oxide-propylene oxide block copolymers having molecular weights between 1,122 and 15,000. Specific examples are the high molecular weight ethoxylated-propoxylated block copolymers ANTAROX® F-88 of 8,082 molecular weight, and ANTAROX® F-108 of 10,812 molecular weight. Among the possible “viscosity improvers” suggested by U.S. Pat. No. 5,707,551 are included the genera of ethoxylated alkyl phenols and ethoxylated aliphatic alcohols. No specific species of these genera are identified.
The second necessary component in U.S. Pat. No. 5,707,551 is an anionic surfactant whose functions are to control the viscosity increase caused by the first component and to adhere to the surface of the suspended solids.
The third necessary component is a bulky nonionic surfactant having a large hydrophobic substituent group. Such materials are known to provide steric interference that inhibits settling.
A fourth optional component in U.S. Pat. No. 5,707,551 is water. The water may be added to adjust the temperature coefficient of solubility of the suspended solids.
As noted above, U.S. Pat. No. 5,906,962 describes a similar surfactant system that includes yet an additional component consisting of a hydratable polysaccharide.
It is evident that the suspension systems described by U.S. Pat. Nos. 5,707,551 and 5,906,962 are complex, require much time and effort to adjust for each suspended solid, and are costly to manufacture and control. A need exists for a non-aqueous, water-soluble suspension system for agricultural solids that is simple, economic and easy to adjust for different suspended solids. In particular, a need exists for simple, low cost, stable, concentrated suspensions of ammonium sulfate that are readily dispersible in water and for other purposes.